The Food Lab: Here's Why McDonald's Burgers Don't Rot

Like Rolling Stones farewell tours or the McRib, every few years they seem to come back. I'm talking about those ready-made-for-reposting, viral-friendly photographs of McDonald's food that doesn't seem to ever rot. It started back in 2008, when Karen Hanrahan shared a photograph of a McDonald's hamburger she bought in 1996 on her blog, Best of Mother Earth. Thing is, she took that photo in 2008, and the 12-year-old burger looked nearly identical to a fresh McDonald's burger. She then went on to explain that the reason the burger didn't rot was because it had "absolutely no nutrition" and was a "chemical food."

Since then, the experiment has been repeated many times, with the photographs dutifully posted online, most recently in the form of a six-year-old Happy Meal on Facebook. Every time, these photographs get shared like crazy and help us all make better food choices.

Or do they?

Surely a "real" hamburger should mold, get eaten by bugs, or otherwise decay, right?

Not so fast. Science requires well-planned experimentation, with multiple data points and controlled variables, to get at the truth of what's really going on. When I first started seeing these postings back in 2010, no such controlled experiments existed, so I set out to remedy that, subjecting my apartment (and wife) to several months of burgers in various stages of decay (or non-decay) to get some answers. You can read up on the details of how I set up the experiments and the conclusions I drew from them.

Long story short: McDonald's burgers don't rot because they dry out, that's it. If you think a McDonald's burger that doesn't rot is unnatural and gross, you should also count saltine crackers, beef jerky, hardtack, croutons, dried beans, or pretty much any nutrient-rich, shelf-stable food in your pantry as unnatural and gross. The reason a McDonald's burger doesn't rot has nothing to do with chemicals, lack of nutrition, or anything else you should be scared of. It all comes down to water activity.

See, a McDonald's hamburger is small and thin, giving it a very high ratio of surface area to volume. It is cooked well-done on a very hot griddle. These factors contribute to rapid moisture loss, resulting in a burger that dries out long before it can start to rot. Moreover, the burgers are cooked in a food-safe environment to a very high temperature that kills any bacteria, and are thus relatively free of any agents of decay to begin with.

Month-old McDonald's and homemade burgers in various configurations.

In my experiments, I cooked homemade burgers of identical shape and size, which showed that this phenomenon is not unique to McDonald's hamburgers. Any burger of the same size and same shape will desiccate and refuse to rot in the exact same way.

Larger burgers from McDonald's, like Quarter Pounders, will start to mold before they completely dry out. Similarly, if you store a McDonald's burger in an environment where moisture is retained (such as in a zipper-lock bag), it will also mold, identically to a homemade hamburger.

For the record, since performing this experiment at home, I've subsequently spoken with researchers who have re-created the testing in an actual food laboratory with lab-grade equipment and controls. The results were exactly the same.

Now, I am no defender of McDonald's, and there are plenty of reasons to avoid eating there too regularly, but, as I've said before, I've got no beef with their beef. My beef is with bad science.

So the next time you see one of those burgers-that-don't-rot photos, maybe think twice about hitting the drive-thru, but think thrice before hitting that "share" button.

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J. Kenji López-Alt is the Chief Culinary Advisor of Serious Eats, and author of the James Beard Award-nominated column The Food Lab, where he unravels the science of home cooking. A restaurant-trained chef and former Editor at Cook's Illustrated magazine, his first book, The Food Lab: Better Home Cooking Through Science is a New York Times Best-Seller, the recipient of a James Beard Award, and was named Cookbook of the Year in 2015 by the International Association of Culinary Professionals.

He's currently raising a daughter by day, writing his second book by night (Now with 10% more science!), and is working on Wursthall, a beer hall in downtown San Mateo which will be open by the end of 2017.

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